It is desirable to increase the proportion of filler in paper products and thereby to reduce the use of fibres. In addition to low price and good availability, fillers also increase the printability and optical properties of paper. However, increasing the filler contents above 30 wt-% in the paper product is challenging, especially when the paper product is a low-weight paper grade. Poor retention of the filler in the paper results in increased contents of the filler in circulation waters of the papermaking process, which may cause problems in the process. Use of chemical retention aids, such as c-PAM (cationic polyacrylamide) has its upper limits too.
Another problem related to the large filler proportions is the weakening of the mechanical properties of the paper product, because the fillers interfere with the bonds between the fibres which create the structural integrity of the paper product mainly by means of hydrogen bonds between cellulose molecules. Both the poor retention of the filler and weakened mechanical properties of the paper product are due to poor fiber-filler bond in the fibrous network.